Seismic phenomena associated with the 1996 Vatnajökull eruption, central Iceland

During late September 1996, a major eruption took place at the NW part of the Vatnajökull glacier in central Iceland. The eruption was preceded by intense seismic activity, which began with a M_w = 5.6 earthquake two days previously. Two very active volcanic systems, Bárdarbunga and Grimsvötn, are situated in that area underneath the permanent ice cap. The volcanoseismic phenomena associated with the eruption were recorded on both temporary (HOTSPOT) and permanent (SIL) seismic networks, covering most parts of the country. The recorded events were categorised, according to their waveform shape and frequency content, into three groups: (1) low-frequency events; (2) mixed-frequency events; and (3) volcanic tremor. The large earthquake at Bárdarbunga volcano, which initiated the seismic activity before the eruption, was located inside the caldera and had the characteristics of a non-double couple event. The epicentres of the earthquake swarm that followed it initially delineated the caldera rim and then migrated towards Grimsvötn, possibly indicating lateral movements of magma from a shallow chamber beneath Bárdarbunga. The eruption affected an area much larger than that between these two volcanoes, since seismic activity was also observed at distances 20 km away, at the Tindafjallajökull volcanic system. The spectral analysis of tremor, recorded at the nearest station to the eruption site, revealed its existence before the onset of the eruption in five narrow frequency bands (0.5-0.7, 1.6, 2.2, 2.8, 3.2 Hz) representing fundamental frequencies with their half- and quarter-subharmonics. This pattern continued until the last day of the eruption. It is believed that the eruption was caused by a dyke injection that had been going on beneath the Vatnajökull area for a period of 10 years.